rotational gravity

[gobblegobble]

we were taught at school that gravity was caused by the earths spin. so is gravity caused by rotation different to the orbit of the earths??

 

thanks in advance

[gobblegobble]

btw i am only 13 sorry if i should know this by now

[YT2095]

Gravity will always be where there are 2 masses, spinning or otherwise, the Earths spin has an opposing effect on gravity, infact if it were to spin much faster we`de all fly off into space

it`s the ballance between spin and gravity that keeps objects in orbit. too fast, they move apart, too slow and they fall together.

hope that helps a little?

[gobblegobble]

how much faster and could it ever happen.... also would it be possible to lower the speed of the earths spin and make gravity that little bit less strong

 

what you said wasn't what i ment really but thanks for the info. its like space walks in the orbit of the earth, the space craft goes along at the same speed as the person but this opposes newtons theaorey right? because it suggests that gravity knows how much mass it has to pull and can pull harder on certain things in the same envireonment. so this comes to the conclusion that it is actually a 4th dimensional dip in space caused by matter... in short the more matter the stronger the gravity. is this differnt for the gravity caused by the spin of the earth??

[YT2095]

the more MASS the stronger the pull, In space or free fall we are free of the effects of gravity, also in space there is no weight, we only have mass, and inertial velocity.

Newtons laws are obeyed perfectly in space, there is no contradiction?

the greater the velocity the more MASSIVE it becomes.

no, I can`t envisage a way that the Earths rotation could be mabe faster, SLOWER yes, infact we have already slowed it down a fraction by building the huge water reservior dams.

the space craft and man would be moving approx 29,000 Km per hour, but so would the man

that`s why when your in a car doing 70 miles an hour and drop a ball, it doesn`t stick to the back window

[JaKiri]

Gravity is not caused by the spin of the earth.

 

Gravity is caused by the interaction of two masses.

 

On the other hand, you can simulate gravity by standing on the inside of a rotating cylinder, like the Wall of Death they have at fairgrounds. What happens then is that your body is moving, and wants to keep moving in a straight line, but the wall forces you to move in a curved path, so you're pressed to the wall as it changes your direction (like in a car, when you turn sharply).

 

The reason that the spaceman and the ship stay close to eachother is that gravity is an acceleration; you accelerate the same no matter what your mass is. Try rolling a full can and an empty can down a slope; they should take the same amount of time to reach the bottom.

 

The stuff about a distortion in 4 dimensional space is Einstein's General Relativity theory. It replaced Newton's theory of gravity after Newton's theories was found to have some things that didn't work (the orbit of mercury sholdn't be like it is if Newton was right), but einstein found a way to fix that.

 

It's immensely complicated*, and for all intents and purposes up until university, you can work with Newtonian gravity.

 

*To quote Lord Rutherford (I think) when he was asked whether it was true if there were only 3 people in the world who understood General Relativity, and then asked what he was thinking about when he didn't reply, 'I was trying to think who the other person is'. (or something along those lines)

 

My next post will be mathematical in basis, it's just explaining what I've said above.

[JaKiri]

Newtonian gravity is expressed in the equation

 

F = GMm/d^2

 

Where F is the force due to gravity, G is a constant to get the units right that we can ignore, M is one mass, m is the other and d is the distance between the object's centers of mass.

 

F = ma

 

This is newton's second law of motion.

 

It states that acceleration is equal to the force divided by the mass; this leads to the common expression of mass as 'resistance to acceleration', because the more mass something has, the harder it is to move it (this is pretty obvious, try and lifting a house; but it's nice when every day things can be explained)

 

ma = F = GMm/d^2

 

a = GM/d^2

 

All that we did was cancel the m's (note, this only works if M is much bigger than m).

 

As you can see, there's no term in there relating in any way to the man or the spaceship's size, so they must bother undergo the same acceleration, and thus appear to not move relative to eachother.

 

As to the spinning...

 

the equation we need is F = mv^2/r, where r is the radius, m is the mass of the object you're spinning and v is the straight line speed it's going at (this is the same speed as a car would go if its wheels were spinning at that rate).

 

The equation describes the amount of force required to keep an object spinning in a circle, given its mass, the radius of the circle and how fast it's going. (The force is pushing into the centre of the circle)

 

Lets now consider what happens when you're standing on a rotating cylinder.

 

Obviously, you're rotating in a circle about the centre, so there must be a force acting on you.

 

This force comes from the floor pressing up at you, and if you spin the cylinder just right, it feels like the floor is pushing you up as much as if gravity was pulling you down. (Remember Newton's famous 'for each action, there is an equal and opposite reaction'? This force (you feel it, say, on your feet when your standing up; it's the floor pushing up at you) is called the reaction force).

 

This is why most people talking about space ships with artificial gravity are talking about making it spiiiiiiiin.

 

Either that or some scifi rubbish

[gobblegobble]

ok fair enough but ur not telling me to forget what i have learned???

 

btw how old is everyone here.... i thought they would be 15/16

[JaKiri]

Everything in schools up unto the age of about 18 is entirely wrong (at least in the sciences).

 

Just learn what they tell you to pass the exams

[gobblegobble]

oh my god!!! do u think i should carry on having an interest in tis stuff or should i quit and wait??

[JaKiri]

Well, if you learn the real stuff, the fake stuff that they teach is much easier; it'll also hold you in good stead for university.

 

Its always good to have interests

[YT2095]

continue! absolutely!

you were curious enough to ask, so it`s in your nature to be interested in such things and that`s comendable!

just learn what your teachers teach, and accept what they say for now, there is a reason they teach it in the order that they do, as each stage is like a stepping stone

no, Don`t forget what they`ve taught you, you`ll need it! it just means that a bit later you will have NEW ideas ADDED to your existing ones

and for what it`s worth, I`m 36.

 

Mrl Jakiri, the cans down the slope will work, but not if one is liquid filled (backed beans or soup) at least it never did with me, cement used to work though a straight drop from a height worked the same though.

[JaKiri]

Liquids are funny like that, they tend to slosh around.

 

Just get two empty ones and put a mass inside one of them.

[YT2095]

yeah, we used cement in the end

it`s a bit like that test to determine which of 2 eggs is the boiled one, that one that won`t spin properly is unboiled.

 

(sorry if off topic a bit)